In electrophotography an image comprising an electrostatic field pattern, usually of non-uniform strength (also referred to as an electrostatic latent image), is formed on an insulative surface of an electrophotographic element comprising at least a photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on the insulative surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise radiation-induced dissipation of the initially uniform electrostatic field is brought about by the creation of electron/hole pairs, which are generated by a material (often referred to as a charge-generation or photoconductive material) in the electrophotographic element in response to exposure to the imagewise actinic radiation. Depending upon the polarity of the initially uniform electrostatic field and the types of materials included in the electrophotographic element, part of the charge that has been generated, i.e., either the holes or the electrons, migrate toward the charged insulative surface of the element in the exposed areas and thereby cause the imagewise dissipation of the initial field. What remains is a non-uniform field constituting the electrostatic latent image.
An especially useful photogeneration material is a heterogeneous or aggregate photoconductive material of the type described in Light U.S. Pat. No. 3,615,414; Kryman et at U.S. Pat. No. 3,679,406, and Contois U.S. Pat. No. 4,350,751. Such materials are well known in the art and typically comprise a co-crystalline complex (aggregate) of at least one sensitizing dye and at least one film-forming aggregating polymer which complex is visible under magnification and is randomly distributed throughout the charge-generation layer.
Electrophotographic elements also contain a material which facilitates the migration of generated charge toward the oppositely charged surface in imagewise exposed areas in order to cause imagewise field dissipation. Such material is often referred to as a charge-transport material, as described, for example, in Hung et al U.S. Pat. No. 4,666,802 and Staudenmayer et al U.S. Pat. No. 4,719,163.
One type of well-known charge-transport material comprises a triarylamine which is a chemical compound containing at least one nitrogen atom that is bonded by at least three single bonds directly to aromatic rings or ring systems. The aromatic rings or ring systems can be unsubstituted or can be further bonded to any number and any types of substituents. Such triarylamines are well known in the art of electrophotography to be very capable of accepting and transporting charges generated by a charge-generation material.
Among the various known types of electrophotographic elements are those generally referred to as multiactive elements (also sometimes called multilayer or multi-active-layer elements). Multiactive elements are so named, because they contain at least two active layers, at least one of which is capable of generating charge in response to exposure to actinic radiation and is referred to as a charge-generation layer (hereinafter referred to as a CGL), and at least one of which is capable of accepting and transporting charges generated by the charge-generation layer and is referred to as a charge-transport layer (hereinafter referred to as a CTL). Such elements typically comprise at least an electrically conductive layer, a CGL and a CTL. Either the CGL or the CTL is in electrical contact with both the electrically conductive layer and the remaining CGL or CTL. Of course, the CGL comprises at least a charge-generation material (a photoconductor); the CTL comprises at least a charge-transport material; and either or both layers may additionally comprise a film-forming polymeric binder. Typical multiactive elements are described in the aforementioned Hung et al and Staudenmayer et al U.S. Patents.
Among the known multiactive electrophotographic elements, are those which are particularly designed to be reusable and to be sensitive to imagewise exposing radiation falling within the visible region of the electromagnetic spectrum. Reusable elements are those that can be practically utilized through a plurality (preferably a large number) of cycles of uniform charging, imagewise exposing, development and/or transfer of electrostatic latent image or toner image, and erasure of remaining charge, without unacceptable changes in their performance. For example, some reusable mutliactive electrophotographic elements that are designed to be sensitive to visible radiation are those in which the CGL contains an aggregate photoconductive material and the CTL contains a triarylamine charge-generation material, as described, for example, in Berwick et al U.S. Pat. No. 4,175,960.
Elements containing such components fairly adequately perform their intended functions and, in the case of the elements described in the aforementioned Berwick et at U.S. patent, have some very important advantages over other known elements. However, there is a significant drawback associated with such elements. For example, a problem can occur when the CTL has been adventitiously exposed to light comprising significant radiation of a wavelength less than about 400 nanometers, i.e., ultraviolet radiation. Such radiation forms a significant portion of the radiation emitted by typical fluorescent room lighting. The problem can occur, for example, when an electrophotographic element is incorporated in a copier apparatus and is exposed to typical room illumination during maintenance or repair of the copier's internal components. The problem, which in commonly referred to as "light fatigue" is manifested as a loss in charge acceptance in subsequent charging cycles of the electrophotographic element as the elment is exercised through its normal cycles of electrophotographic operation after having been adventitiously exposed to light comprising ultraviolet radiation.
For example, in normal cycles of operation such an element might be initially uniformly charged to a potential of about -500 volts, and it might be intended that the element should then discharge, in areas of maximum exposure to normal imagewise actinic visible exposing radiation, to a potential of about -100 volts, in order to form the intended latent electrostatic image. However, if the electrophotographic element has been adventitiously exposed to light comprising ultraviolet radiation, there is a precipitous decrease in the initial potential with an accompanying loss in image quality during normal electrophotogrpahic operation. For example, after such exposure that initial potential may drop 30 to 40 volts or even more. The electrophotogrpahic element may gradually return to its original behavior, e.g., accept an initial charge of -500 volts, but this occurs only after repeated cycling, e.g., 1000 cycles or after standing for several hours depending upon the duration of exposure to such radiation. It is obvious therefore, that it would be advisable to avoid or minimize the light fatigue problem in multiactive elements of the type described hereinbefore.